Field of the Invention and its Purpose
Present-day mechanical salt mining operations are typically conducted by sequential work face drilling; blasting; and broken salt removal operations. However, on occasion the blasting operations have been known to release pressurized fluid and/or ignite combustible liquid or gas accumulations existing behind the working face with disastrous results to the mining crew. Such accumulations may occur in cavities, or in fault fissures, or in porous stria; or in the form of hydrocarbon inclusions occurring along with recrystallized salt fillings of previously formed crevices or cavities in the salt deposit.
Such anomalies are for example especially frequent within a salt "dome"; and it is believed that these are due to the fact that following upthrust formation of the dome and progressive emergence of the enlarged upper portion thereof through a decreasing pressure gradient in the country rock, the peripheral compression forces of the country rock against the dome correspondingly relax. Also, incident to upthrusting formation of the salt dome, the peripheral walls of the surrounding country rock are lifted and inclined upwardly towards the dome, thereby providing permeable zones in the country rock around the dome into which liquid and gaseous hydrocarbons tend to accumulate. Many successful oil and/or gas producing wells are based upon such accumulations.
Although the solid salt per se of such deposits is impermeable to transmission of gas/liquid materials therethrough, it is known that faults, fractures, and open fissures do occur in such domes as explained hereinabove. Such passageways usually extend in vertical or near vertical attitudes and in plan view predominantly either concentrically or straight-line transversely of the dome; and it is found in some cases that hydrocarbon liquids/gases originally occurring in the circumjacent country rock (being squeezed under tremendous pressures) have migrated/infiltrated and/or settled into these fissures or conduits within the body of the salt dome.
This invention relates to a method of insuring/creating a safe operational environment within typical mechanically (dry) mining salt deposit operations; and more particularly to a method for more safely mining such salt deposits having hazardous fluid accumulations occasionally entrapped therein; such as occur by way of example in the "dome" or "diapir" type salt deposits of the Texas-Louisiana Coastal Basin, and/or in the Gulf region of the United States and Mexico. It is also to be understood, however, that the method of the invention is similarly applicable to the mining of potash, trona, and other such evaporite deposits.
The primary object of the invention is to provide a method which will preclude inadvertent triggering of disastrous explosions and/or "blow-outs" from the mine working face with consequent loss of life such as for example was recently experienced in a Louisiana State salt dome mining operation. Incidentally thereto, the invention also contributes to the fracturing and/or "dilation" to a lesser degree of the solid salt mass ahead of the mine face which is to be mined; thereby reducing the powder consumption cost of the subsequent mining operation.